1. Field of the Invention
The present invention relates to telemetric systems communicating data through noisy transmission channels. The present invention particularly relates to telemetric systems in formation evaluation or borehole telemetry. More specifically it relates to the identification and suppression of noise in such channels.
2. Description of the Related Art
In the development, completion, and operation of natural hydrocarbon reservoirs, various telemetric systems and techniques are known and employed to achieve what is known in the art as measurement while drilling (MWD).
For the purpose of this application, MWD includes any type of data transmission from sensor units in the drill bit, bottom hole assembly, or any other part of the sub-surface drill string. Another acronym often encountered in the art besides MWD is LWD (Logging While Drilling). MWD includes in particular low data bit rate transmission systems, as operating below 10 kHz, preferably below 1 kHz, such as acoustic telemetry through the drill string itself, or mud pulse telemetry.
In the latter, currently prevailing technique data are transmitted by means of a mud pressure pulse generator located inside the drill string. The system generates pressure pulses in the drilling fluid or mud, typically by way of a valve or siren type of device. The pulses are detected at the surface by suitable means, e.g., pressure sensors, strain gages, accelerometers, and the like, which means are in general directly attached to the drill string.
Borehole or wellbore telemetry is a well established technology. Improvements to this technology as have been made over the past decades are published for example in a large number of patents, including U.S. Pat. Nos. 3,790,930; 3,820,063; 4,739,325; and 4,932,005.
Of particular interest for the scope of the present invention are the numerous attempts being made to improve the data detection of the transmitted data at the surface. It should be noted that the drilling process presents an exceedingly noisy environment for telemetry owing to the mechanical generation of broadband noise and to the drilling fluid circulation system.
To improve the signal-to-noise ratio, the data as gathered by the sensor units can be encoded such that the distortion by noise has less impact on the data recovery. Usually employed encoding schemes include Frequency Shift Keying (FSK), Phase Shift Keying (PSK) or m-ary pulse coding. Alternatively a binary non return to zero coding may be used. Different encoding methods are described for example in U.S. Pat. Nos. 3,789,355 or 4,562,559.
In U.S. Pat. No. 5,381,092 the signals from those sensors which evaluate the earth formation are subdivided prior to transmission into a plurality of groups, each group represented by one value.
In U.S. Pat. 5,055,837 an attempt is described to improve the quality of the transmission by determining a transfer function which characterizes the transmission properties of the drilling fluid column in the drill pipe.
In an acoustic telemetry system, as described in U.S. Pat. No. 5,128,901, the data signals are (pre-)conditioned to counteract distortions caused by the drill string.
A filtering technique to cancel or minimize noise in the transmitted data signals is disclosed in U.S. Pat. No. 4,878,206. This known approach uses independent measurements of the vibrations of the drill string at the surface to remove pressure disturbance caused by these vibrations and affecting the mud column pressure. A similar technique is known from U.S. Pat. No. 5,289,354.
Probably the closest prior art is an undocumented, "manual" approach to noise filtering, wherein an experienced operator searches the frequency spectrum of the transmitted signal for obvious peaks, marks those frequencies at which the peaks occurs and selects appropriate filters to remove them from the signal. This procedure depends on the quality of the operator. It can not be used to provide a deterministic, reliable, and real-time operation service.
A straightforward automation of the above process faces difficulties arising from the nature of wellbore telemetry signals. Those signals are characterized by a low S/N ratio, large fluctuations in the absolute value of the signal and drifts or baseline shifts.
In view of the above cited prior art it is an object of the invention to provide an automated telemetry system with improved noise reduction and cancellation. The system should be compatible with or independent from the various transmission media and encoding methods. It is a particular object of the invention to provide such a system for downhole telemetry in the low frequency domain, in particular for mud pulse telemetry.